1. Field of the Invention
The present invention relates to a piezoelectric element in which piezoelectric material layers and electrode layers are alternately formed, and the present invention further relates to a method of manufacturing the piezoelectric element.
2. Description of a Related Art
Multilayered structures in each of which insulating (dielectric) layers and electrode layers are alternately formed are utilized for various uses such as multilayered capacitors, piezoelectric pumps, piezoelectric actuators and ultrasonic transducers. In recent years, with the developments of MEMS (micro electro-mechanical systems) related devices, elements each having such a multilayered structure have been microfabricated still further and packaged more densely.
In microfabrication of an element having opposed electrodes, the smaller the area of the element is made, the smaller the capacity between the electrodes becomes, and therefore, a problem occurs that the electrical impedance of the element rises. For example, when the electrical impedance rises in a piezoelectric actuator, the impedance matching can not be taken with a signal circuit for driving the piezoelectric actuator and power becomes difficult to be supplied, and thereby, the performance as the piezoelectric actuator is degraded. Alternatively, in an ultrasonic transducer using a piezoelectric element, detection sensitivity of ultrasonic wave is dropped. Accordingly, in order to enlarge the capacity between electrodes while microfabricating the element, plural piezoelectric material layers and plural electrode layers are alternatively stacked. That is, the capacity between electrodes of the entire element can be made larger by connecting the stacked plural layers in parallel.
As a method of manufacturing a piezoelectric element having a multilayered structure, conventionally, a bulk method employing bulk piezoelectric materials has been known. In the bulk method, a technique of alternately stacking the bulk piezoelectric materials that have been cut in desired thicknesses and electrode layers, and securing them with an adhesive or bolts is used. However, this technique is generally used for manufacturing a relatively large piezoelectric element and not suitable for manufacturing a minute piezoelectric element for the following reason. That is, because piezoelectric materials that have been cut into thin pieces are brittle and easy to break, and the handling of them is difficult. Especially, the piezoelectric material having a thickness of 100 μm or less is easy to break. Further, because the microfabrication is difficult by the technique, the manufacturing process becomes complicated. Furthermore, with respect to finished products, there is a problem about attachment performance due to the adhesive, and a problem occurs that stress is produced in the bonded part. Accordingly, the manufacturing yield is reduced and the cost of manufacturing is increased by the technique, and therefore, the method is not suitable in view of productivity.
Then, manufacturing the piezoelectric element by using a film forming technology such as a green sheet method has been under study.
In the green sheet method, a green sheet (piezoelectric material sheet) is used, which is formed by a mixture of a powder of a piezoelectric material having no plasticity with a binder of an organic material or the like. In this technique, plural piezoelectric material sheets, onto which paste of an electrode material is applied by screen printing or the like, are stacked and baked at high temperature of about 1000° C. so as to allow the binder to fly from the piezoelectric material sheets, and thereby, the piezoelectric materials are made into strong films.
In a piezoelectric element manufactured using such a technique, in order to connect the plural internal electrode layers to each other, interconnection is performed on the side surfaces of the piezoelectric element. FIG. 9 is a sectional view for explanation of a general interconnecting method of a piezoelectric element having a multilayered structure. The piezoelectric element 100 includes plural piezoelectric material layers 101, plural internal electrode layers 102 and 103, and side electrodes 104 and 105.
The internal electrode layers 102 are formed so that one ends thereof extend to one wall surface of the piezoelectric element, and internal electrode layers 103 are formed so that one ends thereof extend to the other wall surface of the piezoelectric element. Thereby, the internal electrode layers 102 are connected to the side electrode 104 and insulated from the side electrode 105 by insulating regions 106. Contrary, the internal electrode layers 103 are connected to the side electrode 105 and insulated from the side electrode 104 by insulating regions 106. By applying a potential difference between the side electrode 104 and the side electrode 105, a voltage is applied between the internal electrode layers 102 and the internal electrode layers 103, and each piezoelectric material layers 101 disposed therebetween expands and contracts by the piezoelectric effect.
However, in the internal electrode layers 102 and 103, the insulating regions 106, in which no electrode is formed, are provided for insulating the electrode layers from either of the side electrodes. The insulating regions 106 do not expand nor contract even when a voltage is applied between the internal electrode layers 102 and 103. On this account, first regions that expand and contract, and second regions that do not expand and contract exist within the piezoelectric material layers 101, and therefore, there is a problem that stress is concentrated between these regions and they are easy to break.
As a related technology, Japanese Patent Application Publication JP-A-60-128683 discloses a method of manufacturing a long-life multilayered piezoelectric actuator (the second page, FIG. 2). According to JP-A-60-128683, internal electrodes are formed on the entire surface of green sheets mainly composed of a piezoelectric ceramic material, the green sheets are stacked in layers and sintered, machining is performed on both side surfaces of the sintered material, ends of the both side surfaces are insulated with a resin every other layer, and conducting layers are formed on the respective surfaces over the processed material. Thereby, durability can be improved compared to the conventional ones in which electrodes can not be formed all over the surfaces, and further, microfabrication can be performed because alignment is not required for forming internal electrodes partially.
However, the epoxy resin used for insulating the side surfaces of the element has a low withstand voltage, and therefore, the thickness of the resin must be made larger. For example, when the voltage for driving the piezoelectric element is 200V to 300V, the resin is required to have a thickness of 20 μm to 30 μm. In the case where the thickness of the resin is large, a problem occurs when an ultrasonic probe is manufactured by arranging plural piezoelectric elements in a one-dimensional or two-dimensional array form. That is, a filling material is poured between the plural piezoelectric elements when the ultrasonic probe is manufactured. However, in the case where gaps between the plural piezoelectric elements arranged are as small as 50 μm, it becomes difficult to pour the filling material into these piezoelectric elements due to the thickness of the resin.